Palaeontologists in South Africa have discovered the fossil of a previously unknown dinosaur dating back 200m years. It was found not on a remote desert plain but in a university storeroom.

The specimen had been collected in the late 1930s and for decades it remained hidden among the biggest fossil collection in the country at Wits University in Johannesburg.

More recently it was considered to represent the remains of another South African dinosaur, Aardonyx. But when palaeontologists Dr Alejandro Otero and Emil Krupandan visited the university to look at early sauropodomorph dinosaurs – mostly herbivores with long necks – they saw the bones did not match.

Observing that one of the specimen’s ankle bones is shaped like a cross, the researchers realised they had a “new” dinosaur on their hands. They named it Sefapanosaurus, after the word “sefapano”, which means “cross” in the Sesotho language indigenous to the area where it was dug up.

Krupandan said: “This find indicates the importance of relooking at old material that has only been cursorily studied in the past, in order to re-evaluate past preconceptions about sauropodomorph diversity in light of new data.”

The announcement was made jointly by researchers from Wits University, the University of Cape Town and Argentina’s Museo de La Plata and Museo Paleontológico Egidio Feruglio.

The remains of Sefapanosaurus – which were unearthed in the Zastron area of what is now Free State province, about 20 miles from the border with Lesotho – include limb bones, foot bones and several vertebrae. It is considered to be a medium-sized sauropodomorph dinosaur, among the early members of the group that gave rise to the long-necked giants of the Mesozoic era.

Dr Jonah Choiniere, co-author and senior researcher in dinosaur palaeobiology at the Evolutionary Studies Institute (ESI) at Wits University, said the Sefapanosaurus had a relatively long neck and small tail but, unlike Brontosaurus, walked on two legs.

“We’re discovering how diverse South Africa was 200m years ago,” he said. “When you look at the fossil record you think of one dinosaur hanging out, but it was probably an ecosystem as diverse as that of mammals today.”

On a tour of the ESI this month, the Guardian observed a wealth of dinosaur remains laid out on laboratory tables while researchers worked painstakingly slowly to extract more fossils from rocks. Choiniere said one and possibly two new discoveries would be announced this year.

Argentinian co-author Dr Diego Pol said Sefapanosaurus and other recent finds in the two countries revealed that the diversity of herbivorous dinosaurs in Africa and South America – when the southern hemisphere continents were a single supercontinent known as Gondwana – was remarkably high in the Jurassic period, about 190m years ago .

Anusuya Chinsamy-Turan, co-author and professor in the biological sciences department at the University of Cape Town, said: “The discovery of Sefapanosaurus shows that there were several of these transitional early sauropodomorph dinosaurs roaming around southern Africa about 200m years ago.”